vllm - 💡(How to fix) Fix [Bug]: Poor Qwen3.5 NVFP4 disagg GSM8K accuracy with 2p1d (2xTEP8 prefill, 1xDEP8 decode)

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Qwen3.5 NVFP4 disaggregated serving with NIXL becomes accuracy-unstable across repeated identical GSM8K runs in the same server process.

Topology:

prefill: 2xTEP8
decode:  1xDEP8
KV cache dtype: bfloat16
prefix caching: disabled
async scheduling: disabled
HMA: enabled
SSM conv state layout: DS

The first 32-question GSM8K run is healthy, but the second identical run in the same job degrades sharply. Logs show successful NIXL transfers and no NIXL transfer failure, compatibility hash mismatch, Traceback, or ERROR.

Error Message

No NIXL transfer failure No compatibility hash mismatch No Traceback / ERROR KV Transfer metrics show successful transfers External prefix cache hit rate ~99.9%

Root Cause

Qwen3.5 NVFP4 disaggregated serving with NIXL becomes accuracy-unstable across repeated identical GSM8K runs in the same server process.

Topology:

prefill: 2xTEP8
decode:  1xDEP8
KV cache dtype: bfloat16
prefix caching: disabled
async scheduling: disabled
HMA: enabled
SSM conv state layout: DS

The first 32-question GSM8K run is healthy, but the second identical run in the same job degrades sharply. Logs show successful NIXL transfers and no NIXL transfer failure, compatibility hash mismatch, Traceback, or ERROR.

Fix Action

Fix / Workaround

============================== CPU Info

Architecture: aarch64 CPU op-mode(s): 64-bit Byte Order: Little Endian CPU(s): 144 On-line CPU(s) list: 0-143 Vendor ID: ARM Model name: Neoverse-V2 Model: 0 Thread(s) per core: 1 Core(s) per socket: 72 Socket(s): 2 Stepping: r0p0 Frequency boost: disabled CPU(s) scaling MHz: 100% CPU max MHz: 3402.0000 CPU min MHz: 81.0000 BogoMIPS: 2000.00 Flags: fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti L1d cache: 9 MiB (144 instances) L1i cache: 9 MiB (144 instances) L2 cache: 144 MiB (144 instances) L3 cache: 228 MiB (2 instances) NUMA node(s): 34 NUMA node0 CPU(s): 0-71 NUMA node1 CPU(s): 72-143 NUMA node2 CPU(s): NUMA node3 CPU(s): NUMA node4 CPU(s): NUMA node5 CPU(s): NUMA node6 CPU(s): NUMA node7 CPU(s): NUMA node8 CPU(s): NUMA node9 CPU(s): NUMA node10 CPU(s): NUMA node11 CPU(s): NUMA node12 CPU(s): NUMA node13 CPU(s): NUMA node14 CPU(s): NUMA node15 CPU(s): NUMA node16 CPU(s): NUMA node17 CPU(s): NUMA node18 CPU(s): NUMA node19 CPU(s): NUMA node20 CPU(s): NUMA node21 CPU(s): NUMA node22 CPU(s): NUMA node23 CPU(s): NUMA node24 CPU(s): NUMA node25 CPU(s): NUMA node26 CPU(s): NUMA node27 CPU(s): NUMA node28 CPU(s): NUMA node29 CPU(s): NUMA node30 CPU(s): NUMA node31 CPU(s): NUMA node32 CPU(s): NUMA node33 CPU(s): Vulnerability Gather data sampling: Not affected Vulnerability Ghostwrite: Not affected Vulnerability Indirect target selection: Not affected Vulnerability Itlb multihit: Not affected Vulnerability L1tf: Not affected Vulnerability Mds: Not affected Vulnerability Meltdown: Not affected Vulnerability Mmio stale data: Not affected Vulnerability Old microcode: Not affected Vulnerability Reg file data sampling: Not affected Vulnerability Retbleed: Not affected Vulnerability Spec rstack overflow: Not affected Vulnerability Spec store bypass: Not affected Vulnerability Spectre v1: Mitigation; __user pointer sanitization Vulnerability Spectre v2: Mitigation; CSV2, but not BHB Vulnerability Srbds: Not affected Vulnerability Tsa: Not affected Vulnerability Tsx async abort: Not affected Vulnerability Vmscape: Not affected

Code Example

==============================
        System Info
==============================
OS                           : Ubuntu 24.04.4 LTS (aarch64)
GCC version                  : (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0
Clang version                : Could not collect
CMake version                : Could not collect
Libc version                 : glibc-2.39

==============================
       PyTorch Info
==============================
PyTorch version              : 2.11.0+cu130
Is debug build               : False
CUDA used to build PyTorch   : 13.0
ROCM used to build PyTorch   : N/A
XPU used to build PyTorch    : N/A

==============================
      Python Environment
==============================
Python version               : 3.12.3 (main, Mar 23 2026, 19:04:32) [GCC 13.3.0] (64-bit runtime)
Python platform              : Linux-6.17.0-1014-nvidia-64k-aarch64-with-glibc2.39

==============================
       CUDA / GPU Info
==============================
Is CUDA available            : True
CUDA runtime version         : 12.8.61
CUDA_MODULE_LOADING set to   :
GPU models and configuration :
GPU 0: NVIDIA GB200
GPU 1: NVIDIA GB200
GPU 2: NVIDIA GB200
GPU 3: NVIDIA GB200

Nvidia driver version        : Could not collect
cuDNN version                : Could not collect
HIP runtime version          : N/A
MIOpen runtime version       : N/A
Is XNNPACK available         : True

==============================
          CPU Info
==============================
Architecture:                            aarch64
CPU op-mode(s):                          64-bit
Byte Order:                              Little Endian
CPU(s):                                  144
On-line CPU(s) list:                     0-143
Vendor ID:                               ARM
Model name:                              Neoverse-V2
Model:                                   0
Thread(s) per core:                      1
Core(s) per socket:                      72
Socket(s):                               2
Stepping:                                r0p0
Frequency boost:                         disabled
CPU(s) scaling MHz:                      100%
CPU max MHz:                             3402.0000
CPU min MHz:                             81.0000
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti
L1d cache:                               9 MiB (144 instances)
L1i cache:                               9 MiB (144 instances)
L2 cache:                                144 MiB (144 instances)
L3 cache:                                228 MiB (2 instances)
NUMA node(s):                            34
NUMA node0 CPU(s):                       0-71
NUMA node1 CPU(s):                       72-143
NUMA node2 CPU(s):
NUMA node3 CPU(s):
NUMA node4 CPU(s):
NUMA node5 CPU(s):
NUMA node6 CPU(s):
NUMA node7 CPU(s):
NUMA node8 CPU(s):
NUMA node9 CPU(s):
NUMA node10 CPU(s):
NUMA node11 CPU(s):
NUMA node12 CPU(s):
NUMA node13 CPU(s):
NUMA node14 CPU(s):
NUMA node15 CPU(s):
NUMA node16 CPU(s):
NUMA node17 CPU(s):
NUMA node18 CPU(s):
NUMA node19 CPU(s):
NUMA node20 CPU(s):
NUMA node21 CPU(s):
NUMA node22 CPU(s):
NUMA node23 CPU(s):
NUMA node24 CPU(s):
NUMA node25 CPU(s):
NUMA node26 CPU(s):
NUMA node27 CPU(s):
NUMA node28 CPU(s):
NUMA node29 CPU(s):
NUMA node30 CPU(s):
NUMA node31 CPU(s):
NUMA node32 CPU(s):
NUMA node33 CPU(s):
Vulnerability Gather data sampling:      Not affected
Vulnerability Ghostwrite:                Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Not affected
Vulnerability Spectre v1:                Mitigation; __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; CSV2, but not BHB
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected

==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.6.8.post1
[pip3] mypy_extensions==1.1.0
[pip3] numpy==2.3.5
[pip3] nvidia-cublas==13.1.0.3
[pip3] nvidia-cuda-cupti==13.0.85
[pip3] nvidia-cuda-nvrtc==13.0.88
[pip3] nvidia-cuda-runtime==13.0.96
[pip3] nvidia-cudnn-cu13==9.19.0.56
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft==12.0.0.61
[pip3] nvidia-cufile==1.15.1.6
[pip3] nvidia-curand==10.4.0.35
[pip3] nvidia-cusolver==12.0.4.66
[pip3] nvidia-cusparse==12.6.3.3
[pip3] nvidia-cusparselt-cu13==0.8.0
[pip3] nvidia-cutlass-dsl==4.4.2
[pip3] nvidia-cutlass-dsl-libs-base==4.4.2
[pip3] nvidia-ml-py==13.595.45
[pip3] nvidia-nccl-cu13==2.28.9
[pip3] nvidia-nvjitlink==13.0.88
[pip3] nvidia-nvshmem-cu13==3.4.5
[pip3] nvidia-nvtx==13.0.85
[pip3] pyzmq==27.1.0
[pip3] torch==2.11.0+cu130
[pip3] torch_c_dlpack_ext==0.1.5
[pip3] torchaudio==2.11.0+cu130
[pip3] torchvision==0.26.0+cu130
[pip3] transformers==5.6.2
[pip3] triton==3.6.0
[conda] Could not collect

==============================
         vLLM Info
==============================
ROCM Version                 : Could not collect
vLLM Version                 : 0.19.2rc1.dev215+g32e45636e (git sha: 32e45636e)
vLLM Build Flags:
  CUDA Archs: Not Set; ROCm: Disabled; XPU: Disabled
GPU Topology:
        GPU0    GPU1    GPU2    GPU3    NIC0    NIC1    NIC2    NIC3    NIC4    NIC5    NIC6    NIC7    CPU Affinity    NUMA Affinity   GPU NUMA ID
GPU0     X      NV18    NV18    NV18    NODE    NODE    NODE    NODE    SYS     SYS     SYS     SYS     0-71    0              N/A
GPU1    NV18     X      NV18    NV18    NODE    NODE    NODE    NODE    SYS     SYS     SYS     SYS     0-71    0              N/A
GPU2    NV18    NV18     X      NV18    SYS     SYS     SYS     SYS     NODE    NODE    NODE    NODE    72-143  1              N/A
GPU3    NV18    NV18    NV18     X      SYS     SYS     SYS     SYS     NODE    NODE    NODE    NODE    72-143  1              N/A
NIC0    NODE    NODE    SYS     SYS      X      NODE    NODE    NODE    SYS     SYS     SYS     SYS
NIC1    NODE    NODE    SYS     SYS     NODE     X      NODE    NODE    SYS     SYS     SYS     SYS
NIC2    NODE    NODE    SYS     SYS     NODE    NODE     X      PIX     SYS     SYS     SYS     SYS
NIC3    NODE    NODE    SYS     SYS     NODE    NODE    PIX      X      SYS     SYS     SYS     SYS
NIC4    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS      X      NODE    NODE    NODE
NIC5    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS     NODE     X      NODE    NODE
NIC6    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS     NODE    NODE     X      PIX
NIC7    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS     NODE    NODE    PIX      X

Legend:

  X    = Self
  SYS  = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
  NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
  PHB  = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
  PXB  = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
  PIX  = Connection traversing at most a single PCIe bridge
  NV#  = Connection traversing a bonded set of # NVLinks

NIC Legend:

  NIC0: mlx5_0
  NIC1: mlx5_1
  NIC2: mlx5_2
  NIC3: mlx5_3
  NIC4: mlx5_4
  NIC5: mlx5_5
  NIC6: mlx5_6
  NIC7: mlx5_7

==============================
     Environment Variables
==============================
LD_LIBRARY_PATH=/home/aperevedents/storage/cuda/cuda128/lib64:
CUDA_HOME=/home/aperevedents/storage/cuda/cuda128
CUDA_HOME=/home/aperevedents/storage/cuda/cuda128
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
TORCHINDUCTOR_CACHE_DIR=/tmp/torchinductor_aperevedents

---

prefill: 2xTEP8
decode:  1xDEP8
KV cache dtype: bfloat16
prefix caching: disabled
async scheduling: disabled
HMA: enabled
SSM conv state layout: DS

---

Hardware: GB200, 4 GPUs/node
Model: Qwen3.5-397B-A17B-NVFP4
vLLM commit: 24337fb860a89f2087df27b02b7c9620a7fd0093
vLLM version: 0.20.2rc1.dev354+g24337fb86.precompiled
Dynamo: 1.2.0.dev20260429
NIXL: nixl-cu13==1.0.1, nixl==1.0.1

---

prefill: 2 workers, each TP8 + expert parallel
decode:  1 logical worker, DP8 + expert parallel
VLLM_SSM_CONV_STATE_LAYOUT=DS
--kv-cache-dtype bfloat16
--no-enable-prefix-caching
--no-async-scheduling
--no-disable-hybrid-kv-cache-manager
--kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both","kv_load_failure_policy":"fail"}'

---

python tests/evals/gsm8k/gsm8k_eval.py \
  --host http://poly0272 \
  --port 8000 \
  --model Qwen3.5-397B-A17B-NVFP4 \
  --max-tokens 256 \
  --no-stop \
  --num-questions 32 \
  --save-results /tmp/qwen35_nvfp4_2xtep8_1xdep8_bf16kv_debug_gsm8k_32.json \
  --save-details

---

Accuracy: 0.875
Invalid responses: 0.062
Total latency: 82.804 s
Total output tokens: 4534

---

Accuracy: 0.688
Invalid responses: 0.219
Total latency: 4.502 s
Total output tokens: 5876

---

First run:  Accuracy 0.906, Invalid 0.000
Second run: Accuracy 0.344, Invalid 0.375

---

<think>
<think>
<think>
...

---

The trains travel for two days.
On the first day, the trains travel for two days.
On the second day, the trains travel for two days.
...

---

No NIXL transfer failure
No compatibility hash mismatch
No Traceback / ERROR
KV Transfer metrics show successful transfers
External prefix cache hit rate ~99.9%
RAW_BUFFERClick to expand / collapse

Your current environment

<details> <summary>The output of <code>python collect_env.py</code></summary>
==============================
        System Info
==============================
OS                           : Ubuntu 24.04.4 LTS (aarch64)
GCC version                  : (Ubuntu 13.3.0-6ubuntu2~24.04.1) 13.3.0
Clang version                : Could not collect
CMake version                : Could not collect
Libc version                 : glibc-2.39

==============================
       PyTorch Info
==============================
PyTorch version              : 2.11.0+cu130
Is debug build               : False
CUDA used to build PyTorch   : 13.0
ROCM used to build PyTorch   : N/A
XPU used to build PyTorch    : N/A

==============================
      Python Environment
==============================
Python version               : 3.12.3 (main, Mar 23 2026, 19:04:32) [GCC 13.3.0] (64-bit runtime)
Python platform              : Linux-6.17.0-1014-nvidia-64k-aarch64-with-glibc2.39

==============================
       CUDA / GPU Info
==============================
Is CUDA available            : True
CUDA runtime version         : 12.8.61
CUDA_MODULE_LOADING set to   :
GPU models and configuration :
GPU 0: NVIDIA GB200
GPU 1: NVIDIA GB200
GPU 2: NVIDIA GB200
GPU 3: NVIDIA GB200

Nvidia driver version        : Could not collect
cuDNN version                : Could not collect
HIP runtime version          : N/A
MIOpen runtime version       : N/A
Is XNNPACK available         : True

==============================
          CPU Info
==============================
Architecture:                            aarch64
CPU op-mode(s):                          64-bit
Byte Order:                              Little Endian
CPU(s):                                  144
On-line CPU(s) list:                     0-143
Vendor ID:                               ARM
Model name:                              Neoverse-V2
Model:                                   0
Thread(s) per core:                      1
Core(s) per socket:                      72
Socket(s):                               2
Stepping:                                r0p0
Frequency boost:                         disabled
CPU(s) scaling MHz:                      100%
CPU max MHz:                             3402.0000
CPU min MHz:                             81.0000
BogoMIPS:                                2000.00
Flags:                                   fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm jscvt fcma lrcpc dcpop sha3 sm3 sm4 asimddp sha512 sve asimdfhm dit uscat ilrcpc flagm sb paca pacg dcpodp sve2 sveaes svepmull svebitperm svesha3 svesm4 flagm2 frint svei8mm svebf16 i8mm bf16 dgh bti
L1d cache:                               9 MiB (144 instances)
L1i cache:                               9 MiB (144 instances)
L2 cache:                                144 MiB (144 instances)
L3 cache:                                228 MiB (2 instances)
NUMA node(s):                            34
NUMA node0 CPU(s):                       0-71
NUMA node1 CPU(s):                       72-143
NUMA node2 CPU(s):
NUMA node3 CPU(s):
NUMA node4 CPU(s):
NUMA node5 CPU(s):
NUMA node6 CPU(s):
NUMA node7 CPU(s):
NUMA node8 CPU(s):
NUMA node9 CPU(s):
NUMA node10 CPU(s):
NUMA node11 CPU(s):
NUMA node12 CPU(s):
NUMA node13 CPU(s):
NUMA node14 CPU(s):
NUMA node15 CPU(s):
NUMA node16 CPU(s):
NUMA node17 CPU(s):
NUMA node18 CPU(s):
NUMA node19 CPU(s):
NUMA node20 CPU(s):
NUMA node21 CPU(s):
NUMA node22 CPU(s):
NUMA node23 CPU(s):
NUMA node24 CPU(s):
NUMA node25 CPU(s):
NUMA node26 CPU(s):
NUMA node27 CPU(s):
NUMA node28 CPU(s):
NUMA node29 CPU(s):
NUMA node30 CPU(s):
NUMA node31 CPU(s):
NUMA node32 CPU(s):
NUMA node33 CPU(s):
Vulnerability Gather data sampling:      Not affected
Vulnerability Ghostwrite:                Not affected
Vulnerability Indirect target selection: Not affected
Vulnerability Itlb multihit:             Not affected
Vulnerability L1tf:                      Not affected
Vulnerability Mds:                       Not affected
Vulnerability Meltdown:                  Not affected
Vulnerability Mmio stale data:           Not affected
Vulnerability Old microcode:             Not affected
Vulnerability Reg file data sampling:    Not affected
Vulnerability Retbleed:                  Not affected
Vulnerability Spec rstack overflow:      Not affected
Vulnerability Spec store bypass:         Not affected
Vulnerability Spectre v1:                Mitigation; __user pointer sanitization
Vulnerability Spectre v2:                Mitigation; CSV2, but not BHB
Vulnerability Srbds:                     Not affected
Vulnerability Tsa:                       Not affected
Vulnerability Tsx async abort:           Not affected
Vulnerability Vmscape:                   Not affected

==============================
Versions of relevant libraries
==============================
[pip3] flashinfer-python==0.6.8.post1
[pip3] mypy_extensions==1.1.0
[pip3] numpy==2.3.5
[pip3] nvidia-cublas==13.1.0.3
[pip3] nvidia-cuda-cupti==13.0.85
[pip3] nvidia-cuda-nvrtc==13.0.88
[pip3] nvidia-cuda-runtime==13.0.96
[pip3] nvidia-cudnn-cu13==9.19.0.56
[pip3] nvidia-cudnn-frontend==1.18.0
[pip3] nvidia-cufft==12.0.0.61
[pip3] nvidia-cufile==1.15.1.6
[pip3] nvidia-curand==10.4.0.35
[pip3] nvidia-cusolver==12.0.4.66
[pip3] nvidia-cusparse==12.6.3.3
[pip3] nvidia-cusparselt-cu13==0.8.0
[pip3] nvidia-cutlass-dsl==4.4.2
[pip3] nvidia-cutlass-dsl-libs-base==4.4.2
[pip3] nvidia-ml-py==13.595.45
[pip3] nvidia-nccl-cu13==2.28.9
[pip3] nvidia-nvjitlink==13.0.88
[pip3] nvidia-nvshmem-cu13==3.4.5
[pip3] nvidia-nvtx==13.0.85
[pip3] pyzmq==27.1.0
[pip3] torch==2.11.0+cu130
[pip3] torch_c_dlpack_ext==0.1.5
[pip3] torchaudio==2.11.0+cu130
[pip3] torchvision==0.26.0+cu130
[pip3] transformers==5.6.2
[pip3] triton==3.6.0
[conda] Could not collect

==============================
         vLLM Info
==============================
ROCM Version                 : Could not collect
vLLM Version                 : 0.19.2rc1.dev215+g32e45636e (git sha: 32e45636e)
vLLM Build Flags:
  CUDA Archs: Not Set; ROCm: Disabled; XPU: Disabled
GPU Topology:
        GPU0    GPU1    GPU2    GPU3    NIC0    NIC1    NIC2    NIC3    NIC4    NIC5    NIC6    NIC7    CPU Affinity    NUMA Affinity   GPU NUMA ID
GPU0     X      NV18    NV18    NV18    NODE    NODE    NODE    NODE    SYS     SYS     SYS     SYS     0-71    0              N/A
GPU1    NV18     X      NV18    NV18    NODE    NODE    NODE    NODE    SYS     SYS     SYS     SYS     0-71    0              N/A
GPU2    NV18    NV18     X      NV18    SYS     SYS     SYS     SYS     NODE    NODE    NODE    NODE    72-143  1              N/A
GPU3    NV18    NV18    NV18     X      SYS     SYS     SYS     SYS     NODE    NODE    NODE    NODE    72-143  1              N/A
NIC0    NODE    NODE    SYS     SYS      X      NODE    NODE    NODE    SYS     SYS     SYS     SYS
NIC1    NODE    NODE    SYS     SYS     NODE     X      NODE    NODE    SYS     SYS     SYS     SYS
NIC2    NODE    NODE    SYS     SYS     NODE    NODE     X      PIX     SYS     SYS     SYS     SYS
NIC3    NODE    NODE    SYS     SYS     NODE    NODE    PIX      X      SYS     SYS     SYS     SYS
NIC4    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS      X      NODE    NODE    NODE
NIC5    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS     NODE     X      NODE    NODE
NIC6    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS     NODE    NODE     X      PIX
NIC7    SYS     SYS     NODE    NODE    SYS     SYS     SYS     SYS     NODE    NODE    PIX      X

Legend:

  X    = Self
  SYS  = Connection traversing PCIe as well as the SMP interconnect between NUMA nodes (e.g., QPI/UPI)
  NODE = Connection traversing PCIe as well as the interconnect between PCIe Host Bridges within a NUMA node
  PHB  = Connection traversing PCIe as well as a PCIe Host Bridge (typically the CPU)
  PXB  = Connection traversing multiple PCIe bridges (without traversing the PCIe Host Bridge)
  PIX  = Connection traversing at most a single PCIe bridge
  NV#  = Connection traversing a bonded set of # NVLinks

NIC Legend:

  NIC0: mlx5_0
  NIC1: mlx5_1
  NIC2: mlx5_2
  NIC3: mlx5_3
  NIC4: mlx5_4
  NIC5: mlx5_5
  NIC6: mlx5_6
  NIC7: mlx5_7

==============================
     Environment Variables
==============================
LD_LIBRARY_PATH=/home/aperevedents/storage/cuda/cuda128/lib64:
CUDA_HOME=/home/aperevedents/storage/cuda/cuda128
CUDA_HOME=/home/aperevedents/storage/cuda/cuda128
PYTORCH_NVML_BASED_CUDA_CHECK=1
TORCHINDUCTOR_COMPILE_THREADS=1
TORCHINDUCTOR_CACHE_DIR=/tmp/torchinductor_aperevedents
</details>

🐛 Describe the bug

Summary

Qwen3.5 NVFP4 disaggregated serving with NIXL becomes accuracy-unstable across repeated identical GSM8K runs in the same server process.

Topology:

prefill: 2xTEP8
decode:  1xDEP8
KV cache dtype: bfloat16
prefix caching: disabled
async scheduling: disabled
HMA: enabled
SSM conv state layout: DS

The first 32-question GSM8K run is healthy, but the second identical run in the same job degrades sharply. Logs show successful NIXL transfers and no NIXL transfer failure, compatibility hash mismatch, Traceback, or ERROR.

Environment

Hardware: GB200, 4 GPUs/node
Model: Qwen3.5-397B-A17B-NVFP4
vLLM commit: 24337fb860a89f2087df27b02b7c9620a7fd0093
vLLM version: 0.20.2rc1.dev354+g24337fb86.precompiled
Dynamo: 1.2.0.dev20260429
NIXL: nixl-cu13==1.0.1, nixl==1.0.1

Repro

Run disagg server with:

prefill: 2 workers, each TP8 + expert parallel
decode:  1 logical worker, DP8 + expert parallel
VLLM_SSM_CONV_STATE_LAYOUT=DS
--kv-cache-dtype bfloat16
--no-enable-prefix-caching
--no-async-scheduling
--no-disable-hybrid-kv-cache-manager
--kv-transfer-config '{"kv_connector":"NixlConnector","kv_role":"kv_both","kv_load_failure_policy":"fail"}'

Then run the same GSM8K command twice:

python tests/evals/gsm8k/gsm8k_eval.py \
  --host http://poly0272 \
  --port 8000 \
  --model Qwen3.5-397B-A17B-NVFP4 \
  --max-tokens 256 \
  --no-stop \
  --num-questions 32 \
  --save-results /tmp/qwen35_nvfp4_2xtep8_1xdep8_bf16kv_debug_gsm8k_32.json \
  --save-details

Run again with a different output file.

Observed Results

First run:

Accuracy: 0.875
Invalid responses: 0.062
Total latency: 82.804 s
Total output tokens: 4534

Second identical run:

Accuracy: 0.688
Invalid responses: 0.219
Total latency: 4.502 s
Total output tokens: 5876

A previous non-debug run showed an even larger second-run drop:

First run:  Accuracy 0.906, Invalid 0.000
Second run: Accuracy 0.344, Invalid 0.375

Symptoms in generated text

The degraded run contains repeated <think> tokens and repetitive loops until max_tokens, for example:

<think>
<think>
<think>
...

and:

The trains travel for two days.
On the first day, the trains travel for two days.
On the second day, the trains travel for two days.
...

Several bad responses are FINISHED_LENGTH_CAPPED.

Notes

This does not appear to be a transport failure:

No NIXL transfer failure
No compatibility hash mismatch
No Traceback / ERROR
KV Transfer metrics show successful transfers
External prefix cache hit rate ~99.9%

This looks like state/cache pollution or decode-side correctness instability across repeated runs in the same disaggregated server process.

Full logs: 171031.zip

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